Pistachio nuts comprise a greenish edible seed, enclosed within a hard, straw-colored shell (endocarp), enclosed in turn within a fleshy, green, outer hull (pericarp). Pistachio nuts are typically sold as a food product to end consumers within shells, but without hulls. The hulls of pistachio nuts are stronger and more tenaciously adherent to the shells, and thus more difficult to remove than the hulls of many nuts, particularly if the pistachio is not yet ripe. Further, the hulls are preferably removed from the shell promptly after harvest or else the straw color of the shell will become unattractively blemished. Accordingly, removing the hulls of pistachios in a timely manner and in commercial scale quantities requires either abundant manual labour or mechanized methods. A variety of machines for removing the hulls of pistachio nuts have been developed in the prior art.
U.S. Pat. No. 4,034,665 to McFarland et al. describes a two-stage pistachio huller comprising a pair of rollers and a plurality of fine rollers. In the first stage of operation, the pistachio nuts are passed through the pair of rollers to rupture their pericarps and form tag ends of pericarps adherent to their shells. In the second stage of operation, the pistachio nuts are advanced along the longitudinal direction of a plurality of fine rollers that alternately and oppositely oscillate to strip the tag ends of pericarps from the pistachio nuts. During the second stage, nozzles supply pressurized water to moisten and soften the pericarp of the pistachio nuts, rinse the pericarp from the shells of the pistachios, and urge the pericarp fragments and hulled pistachios along the longitudinal direction of the fine rollers. Finally, the pericarps and the hulled pistachios are separated by screening. This pistachio huller suffers from the disadvantage that it may require substantial amounts of water to operate and undesirably moistens the pistachio nuts.
U.S. Pat. No. 4,448,115 to Volk, Sr. describes a pistachio huller comprising a drum with projections on its outer surface, and a semi-flexible, serrated surface having yieldable slots narrower than the pistachio nuts. In operation, pistachio nuts are deposited on the surface so that they rest partially within the slots. The drum is rotated so that the projections urge the pistachio nuts into compressive contact with the serrated surface and the projections, thereby abrading and hulling the pistachio nuts. The pericarps and the hulled pistachios are subsequently separated by blown air. This pistachio huller suffers from numerous potential disadvantages including that the semi-flexible material is prone to wear, and must therefore be monitored and maintained for proper performance.
U.S. Pat. No. 5,245,918 to Volk, Sr. describes a similar pistachio huller comprising a drum with projections on its outer surface, and a rigid concave surface having slots wider than the pistachio nuts to be hulled. In operation, pistachio nuts are deposited on the concave surface so that they fall within the channels. The drum is rotated so that the projections push the pistachio nuts along the channel, thereby abrading and hulling the pistachio nuts. The pericarps and the hulled pistachios are subsequently separated by blown air. This pistachio huller suffers from numerous potential disadvantages including that residues of the pericarp may accumulate within the channels of the pistachio huller, and thereby interfere with its proper operation.
Accordingly, there is a need in the art for an apparatus and method for removing the hulls of pistachio nuts that overcomes the disadvantages of the prior art. The apparatus preferably operates without water assistance, separates the removed hulls from the hulled pistachios without the need for additional mechanisms, and is simple and economical to construct, operate, and maintain.